Vascular plants were the next evolutionary step…Why?


The Devonian Period: First forests

The big picture: Vascular plants now dominate Earth
- Bryophytes ruled for <100 million years
- vascular plants show up ~425mya
- Plants then evolved vascular tissues
- grow taller than bryophytes
- connected new leaves & roots
- First vascular plants lacked seeds
- sperm still need water to swim
- ferns and lycophytes = 1st groups
- Fern and lycophyte diversity now diminished

Give thanks to extinct seedless vascular plants: Coal

Vascular system: the basics
- Xylem = water conducting (one way)
- Phloem = sugar conducting (multi-directional)
- Conducting cells provide biomechanical support
- lignin in cell walls
- does not rot easily
- does not collapse easily
- think bundles of rigid straws or pipes (seedless plants)
- Vascular elements move liquid under tension
- Vascular elements evolved independently several times


Evolution of roots
- Vascular tissues belowground
- extension of ancient stems
- bryophytes have no ‘true’ roots
- Absorb water and nutrients
- Anchor the plant in place
- Fossils show roots in lycophytes ~ 400mya
- convergent evolution in ferns

Evolution of leaves
- Photosynthetic organ on plants
- Microphylls: single unbranched vein
- unique to lycophytes (add on phylogeny)
- Megaphylls: leaves with branched veins
- majority of vascular plants
- Why do more veins matter?

Evolution of leaves: sporophylls
- Sporophyll: modified leaf with sporangium
- vastly different looks
- Trophophyll: vegetative leaf
- Sporagnium: spore container (house)
- ferns: clusters of sporagnia called ‘sori’
- lycophytes: ‘stobilus’ holds many sporagnia (like a cone)
- angiosperms: structures in flowers
- Spore mother cell: cell inside sporagnium
- undergoes meiosis to make spores
- What is the ploidy of each cell type in this picture?

Lycophytes: First vascular plants

- Independent, branched sporophyte
- Lignified vascular tissue, tracheids & roots
- tracheid = special xylem cell
- Leaves are microphylls
- Some with strobili (evolved multiple times)
- Some with heterospory (Selaginella/Isoetes)
- male (micro-) and female (mega-) spores
Ferns
- Whisk Ferns, Equisetum & Ferns
- mostly homosporous
- whisk ferns do not have roots
- Homospory = spores that germinate to produce bisexual (both male and female) gametophytes
- Leaves are megaphylls
- whisk ferns lost leaf (enation)
- Equistem has simplified megaphylls

Ferns
- Whisk Ferns, Equisetum & Ferns
- mostly homosporous
- whisk ferns do not have roots
- Leaves are megaphylls
- whisk ferns lost leaf (enation)
- Equistem has simplified megaphylls
- New Gaga genus with 19 species
- “We wanted to name this genus for Lady Gaga because of her fervent defense of equality and individual expression” Pryer @ Duke University


Homosphorous life cycle (mostly) with dominant sporophytes

The consequences of homospory
- One spore → one gametophyte → bisexual gametophyte
- everything is haploid (1N)
- Produces both sperm and egg via mitosis
- A lot of self fertilization
- What is good/bad about this?

How spore-bearing plants avoid self-fertilization…

- Turn other gametophytes male
- Archegonia (F) and Antheridia (M) mature at different times
Seedless vascular plants have free living separate generations
- Sporophyte: The fern you see in the woods
- Gametophye: independent haploid generation
- start out female or bisexual
- Derived Trait - remember how a moss looks!
- What does natural selection favor diploid dominant lifecycles?
